NEW EVIDENCE ON THE MECHANISM OF RADIATION LEUKAEMOGENESIS 51 



dose of radiation, it appears (see Fig. 1) that there is a threshold dose for 

 radiation alone, but apparently not for radiation followed by urethane 

 treatment. 



Assuming that for initiating action, radiation is equally effective whether 

 it is given as a single dose or in divided doses (for which, therefore, there would 

 be no threshold dose) whereas for jjronioting action (which is known to be a 

 very slow process), radiation is relatively ineffective when given once only, but 



35 



Radiation alone 



Radiation followed by urethane "<" 



31% (47/152) 



°''i5%(l2/8l) 



o 77.(7/97) 



50 100 ISO 200 250 300 350 400 450 

 Radiation dose (r) 



Fig. 1. Incidence of leukaemia in C57BL mice treated with single doses of X-rays, at 

 different dose levels, with and without subsequent injections of urethane. 



very effective when given in divided doses over a period of weeks, then an 

 explanation would be found both for the conflicting evidence about the 

 existence or non-existence of a threshold dose for leukaemogenesis as a whole, 

 and also for the difference in effectiveness between single and divided doses of 

 radiation for leukaemogenesis. 



We realize, of course, that further data, based on very much larger 

 numbers of animals, would be needed before this highly speculative explana- 

 tion could be accepted. But as a working hypothesis, it may serve the purpose 

 of stimulating further experimentation on the subject. 



REFEEENCES 



Berenblum, I. (1941). Cancer Res. 1, 807. 

 Bebenblum, I. (1954). Cancer Res. 14, 471. 

 Berenblum, I. (1960). Med. J. Australia ii, 721. 

 Berenblum, I., and Haran, N. (1955). Cancer Res. 9, 268 

 Berenblum, I., and Trainin, N. (1960). Science 132, 40. 



Berenblum, I., and Trainin, N. (1961a). Proc. 3rd Australasian Conf. Radiobiology, 

 p. 98, Butterworths, London. 



